Use isl::manage_copy to simplify calls to isl::manage(isl_.._copy())

Use isl::manage_copy to simplify calls to isl::manage(isl_.._copy())

As part of this cleanup a couple of unnecessary isl::manage(obj.copy()) pattern
are eliminated as well.

We checked for all potential cleanups by scanning for:

  "grep -R isl::manage\( lib/ | grep copy"

llvm-svn: 325558